It was Leach-McGill's first tweet in 15 months, since he retweeted a pro-Safe Schools tweet on March 3, 2016. His account had only tweeted 23 times.

The tweet went viral on Monday morning, with ABC News, Huffington Post, Junkee and LGBTI media in Australia all reporting on the alleged poster as part of coverage of the upcoming postal survey on same-sex marriage.

The opposition Labor leader Bill Shorten went on to condemn the poster, and prime minister Malcolm Turnbull suggested LGBTI Australians confronted by it should be hugged.

BuzzFeed News has attempted to trace the poster's origins.

Using a reverse image search, the earliest appearance of the poster in question appears to be on Neo-Nazi forum Iron March in March this year.

Variations of the poster on other sites have the Iron March website URL in the bottom left-hand corner. The URL appears to be blacked-out in the photo of the poster reportedly seen in Chinatown.

A US-based user claiming credit for creating the poster has tweeted his joy at seeing the poster being used in Australia.

Leach-McGill's snap of the poster appears to be the only one that has been posted publicly on social media, and no other photos of the poster have so far surfaced online.

Despite all the attention, none of the news outlets that reported on the poster have independently verified that it was ever on display in Melbourne.

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On Monday afternoon, Leach-McGill tweeted that the poster had been removed. Crikey visited the location on Tuesday but could find no trace of it, or the pro-marriage equality posters that were said to have replaced them.

When BuzzFeed News spoke to Melbourne City Council on Monday afternoon and Tuesday morning, a spokesperson said they had not seen the poster, or had any reports of the poster being plastered anywhere in Melbourne.

BuzzFeed News attempted to contact several people who either claimed to have seen the poster, or to have covered it up with pro-marriage equality posters, but none have been able to produce evidence that the poster itself has been seen in Melbourne.

One of the lead campaigners in the "no" side, Australian Christian Lobby managing director Lyle Shelton, was asked about the poster on Ten's The Project on Monday and claimed it was a false flag operation.

"That obviously doesn't come from us, nor would it," he said, suggesting that The Project's Waleed Aly's argument that the "no" side wasn't being respectful might be why the poster was put up.

"I don't know who put that poster up...I suspect it might have been put up by people trying to prove the point you are making."

Getup campaigner Sally Rugg also shared an image of a poster put up over the weekend in Brisbane by a group that describes itself as advocating for "the protection of White Identity and the fundamental Australian values rooted in the Christian European Traditions".

"The destruction of Traditional Marriage, the forced mixing of cultures into our nations, the consistent propaganda perpetuated by inflated facts or down right lies has GOT TO STOP," a post states.

There is currently a Senate committee looking into the postal survey on same-sex marriage. Chair of the committee, Labor senator Jenny McAllister, has asked for such material to be sent to the committee.

UPDATE

Aug. 22, 2017, at 22:15 PM

Wil Stracke, a campaigner for Victorian Trades Hall Council told BuzzFeed News on Tuesday night that organisers had seen and ripped down some of the Melbourne posters.

"One of our organisers spotted some of the posters in the morning. She was pretty angry and upset and tore them down. When we saw them come up on social media, the rest of our team decided to hit the CBD to get rid of them. They found a couple of the posters and put up 'yes' posters in their place. They also ran into a bunch of students who had been out scouring the city and said they had been tearing the posters down," she said.

She was unable to produce any evidence of the posters, however, but said that if any were spotted in the future, then they would keep the evidence. Stracke said that the website the poster came from had a history of doing poster runs at Trades Hall and other progressive organisations and university campuses.

BuzzFeed News has also now heard from Leach-McGill, who sent the same photos already publicly available via direct message.

UPDATE

Aug. 23, 2017, at 22:42 PM

Leach-McGill emailed BuzzFeed News on Wednesday night with a copy of the original photo. There is no location data in the image metadata, but it does confirm that the photo was taken last Thursday, August 17 at 6:22pm.

Leach-McGill revealed it wasn't him who first took the photo, but a friend who snapped it on Thursday night, and sent it to him via text to tweet to the council.

"We talked about it at length on Friday and by Saturday, still angry about the content, decided to tweet it and call it out," he said.

"With my 22(ish) followers (at the time) and seldom used Twitter account, we expected a polite message from Melbourne City Council to say that they'd addressed it or to tell us where to report it. However, by Sunday night it had started getting noticed, retweeted and commented on."